A waterproof structure which is made by melting and spreading a natural asphalt over a substructure to continuously form a membrane thereby forming a watertight layer on the substructure has been used for a long time in Europe. Such a waterproof structure, which is formed of a simple asphalt, cannot accept shrinking, cracking or repetition of such movement caused in the substructure, resulting in breakage of the waterproof structure, and also this structure is defective in its weather-resistant property, that is, it tends to crack or melt when it is subjected to violent changes of temperature. In order to eliminate such disadvantages, an improved waterproof structure has been invented which comprises a sheet-like reinforcement or base material embedded therein.
The reinforced asphalt structure of this kind is made by firstly preparing a felt-like base material of paper fibres, then saturating said base material with molten straight-run asphalt to form a so-called "asphalt-felt" and coating said asphalt-felt by blowing asphalt on both side surfaces of complete an asphalt roofing.
This structure has been further improved by using a base material made of cotton or linen fibres, which improve the strength of the base material, or are made of asbesto fibres, which improve the decomposition-resistant property thereof. This base material is saturated with straight-run asphalt and is coated with blown asphalt, in the same manner as in the asphalt-felt type structure, so that a three-layered structure having a core of the base material is formed.
Any of the felt-like base materials heretofore used has a water-absorbing property and is fabricated in a porous form, so that the waterproof structure making use of such felt-like base material has a defect in its waterproofing property because of the pinholes remaining in the base material layer and/or coating layer. The base material of this kind has further defects in that it is easily broken by shrinking or cracking of the substructure, trembling of a building standing thereon or repeated shearing stress, and any material other than asbesto has a tendency to decomposition.
In order to avoid the above defects, it is naturally conceived to use a base material made of synthetic fibres with several advantages, particularly in the form of a non-woven fabric. The non-woven fabric, if made of staple fibres, presents disadvantages similar to those of the above-mentioned felt-like structure and consequently it has little availability in the asphalt roofing structure. It has been proposed to develop a non-woven fabric of continuous filaments, but such a fabric has not yet been actually employed to make an asphalt roofing.
The non-woven fabric of continuous long filaments can be manufactured by an "adhering" method which comprises properly arranging the filaments of synthetic fibres extruded from a nozzle under the action of mechanical means or flowing air to form a web of randomly stratified synthetic fibres, then applying an adhesive material thereon by spraying or dipping, squeezing out an excess amount of the adhesive material from said web by means of rolls and then drying the same. Alternatively, the non-woven fabric of this kind can be manufactured by a "heat-pressing" method which comprises preparing a web as in the above adhering method and then pressing said web from one or both sides thereof by heating rolls having plain surfaces or heating rolls having a plurality of projections on the surfaces, thereby fusion-bonding the filaments.
In case of the non-woven fabric of adhering type, the fibres are so arranged that they are not directed to only one direction, thereby forming a web in which the fibres are stratified and adhered together at the crossing points thereof. In such a construction of the web, the evaporation of water occurs from the front and rear outside surfaces during the drying step, and the water remaining within the inside of the web is displaced successively under the capillary action toward the front and rear surfaces which contact the atmosphere. When the water is displaced, the adhesive material in or near the front surface is collectively carried by the water to the outside surface. Also the adhesive material in or near the rear surface is subjected to the same action, together with the action of gravity, whereby it is carried to the outside surface. Accordingly, the non-woven fabric thus obtained contains more amount of adhesive material in its front and rear surface regions than in its inside region, and its has been reported that the amount of the adhesive material contained in the surface region is about three times as much as that contained within the inside region of the fabric, resulting in an extremely unbalanced construction.
Now, the adhering structure of fibres constituting the non-woven fabric of this type will be considered. It has been reported that the non-woven fabric generally comprises adhered portions at the ratio of the 29% in which each of pairs of fibres are crossed and adhered together at substantially right angles, 25% in which each pair of fibres are adhered together at their outside surfaces and 46% in which three or more fibres are not crossed at a same point but adhered together at very close points. In case where the adhesive material is used in an amount of more than 6% of that of the fibres, the adhesive material forms films near the crossing points of the fibres, and it has been reported that in an example where the adhesive material has been used in an amount of 28% (by weight) of the fabric the adhesive material in the form of films might occupy about 50% of total surface area of the fibres.
Accordingly, the non-woven fabric of the adhering type is rigid and lacks flexibility, and the asphalt roofing using such non-woven fabric as its base material still presents disadvantages in that it tends to cause a crack when it is folded and it may bring about separations of the adhered portions when it is crumpled so that pinholes or interlayer separations are likely to be produced in the asphalt by the crack or some movement caused in the substructure, and in some cases the asphalt roofing may present substantially the same phenomenon as that of the oldest type base material made of paper felt.
When non-woven fabric of the heat-pressing type is used, the fibres are thermally bonded at their crossing points by the action of the heating rolls from the front side or front and rear sides, or the fibres are spot-bonded together by the action of the projections of the heating rolls. In any case, the fibres must be bonded together by the fusion-bonding action together with the pressing action, so that the non-woven fabric thus formed has a relatively increased fibre density and a rigid paper-like structure.
Thus, although the asphalt roofing making use of this type of non-woven fabric may present a somewhat improved character over that of adhering type in its tendency to crack when folded or with respect to interlayer separation by crumpling, it presents another problem in that the rate of saturation of the fabric with asphalt is decreased because of the increase in the fibre density, so that the fabric must be treated at higher temperatures in order to increase the rate of saturation. Moreover, the asphalt roofing of this type still presents the disadvantage of pinholes being produced in the asphalt by any crack in or some movement of the substructure.
The asphalt roofing making use of either type of the above mentioned non-woven fabrics presents another problem as explained hereunder. The asphalt roofing is manufactured, in the same manner as the paper fibre roofing as aforementioned, by the method comprising two steps of saturating the non-woven fabric with molten asphalt (blowing asphalt having good quality) and then coating the saturated fabric with the same asphalt on its both surfaces. When a waterproof layer is produced by such asphalt roofing, it is necessary to use a molten asphalt at high temperature, for example 240.degree.-250.degree. C, to adhere the asphalt roofing onto a substructure or the asphalt roofing layers one on the other. Since the asphalt roofing of the above type has a relatively small thickness (about 2 mm, including asphalt coating layers on front and rear surfaces), it is wholly softened when it is exposed to such a high temperature, so that it tends to cause wrinkles or folds after cooling. Accordingly, the operation of applying the asphalt roofing requires a skilled worker.
As explained above, the non-woven fabric serves to increase the strength and decomposition-resistant property of the base material of the asphalt roofing but it still presents some disadvantages, and consequently there remains a question of why such a hard paper-like substance must be used as a base material of an asphalt roofing. In other words, the non-woven fabric including loosely interconnected fibres causes deformation of the fabric itself by the contraction or elongation of the fibres when it is exposed to a molten asphalt at high temperature and also it causes deformation by the action of the various rolls required for continuous production line.
It may be conceived to prevent such deformations by increasing the amount of the adhesive material, adding a heat-resistant adhesive material (for example, starch which is a natural, highly-polymerized substance) or compressing the fabric so as to increase the interconnection between fibres, but such measures of preventing the deformation of the fabric may necessarily result in losing the flexibility thereof and producing a more hardened paper-like structure. That is, such a concept is contrary to our invention which aims at providing a suitable base material for an asphalt roofing and a method of making the same without producing deformation of the fabric, in view of the fact that such a concept is primarily directed to the prevention of deformation of non-woven fabric even though as asphalt roofing can be produced therefrom.
Such a hardened non-woven fabric is effective in preventing the deformation, contraction or elongation, which may be caused when it is dipped in a hot molten asphalt, but this fabric cannot accept such severe situations as cracks in or some movement of the substructure, as explained above, in the process for preparing an asphalt waterproof layer in which the watertightness should be obtained by the non-permeability of the asphalt itself, therefore this type of non-woven fabric cannot be considered to provide a good asphalt roofing. For these reasons, an asphalt roofing having such non-woven fabric as its base material has not yet been widely used.
Recently another type of non-woven fabric has been developed. This non-woven fabric is manufactured by a "needle-punching" method which comprises developing long filaments of synthetic fibres, immediately after they have been melt-spun, into fore-and-aft and right-and-left horizontal directions to form a thick web in which the filaments are intertwined with each other, and then applying a needle-punching treatment thereto to cause the fibres to be oriented also in an up-and-down direction, thereby forming a non-woven fabric in which the fibres are oriented three-dimensionally and intertwined with each other in a complicated manner. The non-woven fabric of this type is entirely different from those of the above-mentioned two types, that is, the former is superior to the latter in tensile strength, compression-resistant property, restoring property or the like owing to the complicated and three-dimensionally intertwined structure of the many fibres and, in addition the former has a relatively large thickness and presents a bulky felt-like appearance.
This non-woven fabric has a further advantage in that a tensile force applied to stretch the fabric in right-and-left direction acts to cause the respective fibres in the fabric to come close together and increase the resistance against tensile stress, and further acts to rearrange the randomly arranged horizontal fibres into the stretching direction and draw out the fibres from the needle-punched region, whereby the fabric permits a considerably large elongation before it bcomes broken and easily restores its original shape after the tensile force is removed. Such a property cannot be obtained by the above two types of the non-woven fabrics, and this is superior character of the needle-punching type non-woven fabric.
This non-woven fabric is now widely used in the fields of heat insulation, cushions, filters, oil blotters, carpets and the like because of its flexible and bulky characteristics. However, it has been generally said that this type of non-woven fabric cannot be used as a material for fabricating asphalt roofing because this non-woven fabric contains fibres not adhered together and it is excessively flexible, stretchable and bulky, so that it becomes deformed when it is exposed to the molten asphalt at high temperature. This is a reason why an asphalt roofing having a base material of non-woven fabric of this type has not yet been actually put on the market throughout the world.